The present invention relates to an improved polyolefin composition which produces moldings with improved dimensional stability and mechanical properties, particularly improved weld strength, low warpage, smooth appearance, and improved heat resistance, without impairing the outstanding chemical stability and moldability inherent in polyolefins.
Heretofore, polyolefins have been used as molding materials on account of high tensile strength, stiffness, and heat resistance attributable to high crystallinity and outstanding electrical properties and chemical stability attributable to resin structure. However, much higher stiffness and heat resistance are required for application areas where the moldings are used as mechanical parts or structural member or used in a high-temperature environment. In order to improve the stiffness and heat resistance of crystalline polyolefins, it is known to incorporate an inorganic filler such as talc, mica, and glass fiber. When a polyolefin is incorporated with talc or mica, the resulting composition is not sufficient in stiffness at high temperatures. When a polyolefin or modified polyolefin is incorporated with glass fiber, the resulting composition is greatly improved in stiffness and heat resistance, but suffer from disadvantages. That is, the moldings made therefrom have great warpage due to anisotropic molding shrinkage caused by orientation of glass fiber and have welds with decreased strength. Such welds are easy to break in moldings having self-tapping bosses or openings. In addition, glass fiber appears as streaks on the surface of moldings, making the appearance poor and the surface rough. The rough surface impairs printability and resistance to hot water.
In order to improve the stiffness at high temperature, molding shrinkage, impact strength, and external appearance, there have been proposed several polyolefin compositions incorporated with powdery inorganic compounds such as glass fiber, calcium carbonate, barium carbonate, and "Shirasu" balloon. Known such compositions include a composition of unmodified carboxylic acid-modified polyolefin containing 3 to 40 wt% (based on the composition) of glass fiber and 10 to 30 wt% (based on the composition of an inorganic compound selected from calcium carbonate, barium carbonate, or "Shirasu" balloon, a reinforced polyolefin resin composition comprising glass fiber and non-reinforcing particulate inorganic compound, as disclosed in Japanese Patent Laid Open No. 89448/12975, and a polyolefin composition incorporated with talc powder filler, glass fiber, and synthetic or natural rubber, as disclosed in Japanese Patent Laid-Open No. 136736/1976.
These compositions are superior in warpage and external appearance to a polyolefin composition reinforced with glass fiber alone, and in impact resistance to a polyolefin composition incorporated with rubber. Nevertheless, the stiffness at high temperature, warpage, and weld strength have not been improved sufficiently for many applications.